1. Field of the Invention
This invention relates to a sealed hydraulic lifter and, more particularly, to a hydraulic lifter suitable for use as a lash adjuster in the valve train, or valve drive mechanism, of an internal combustion engine.
2. Description of the Prior Art
A hydraulic lifter for the abovementioned application includes a cylinder member and a hollow, cylindrically shaped plunger member nested slidably within the cylinder member for telescoping motion. The cylinder and plunger members define a pressure chamber between their closed ends. The plunger member is provided internally with a reservoir chamber for receiving leakage oil from the pressure chamber through a clearance space which exists between the telescoping cylinder and plunger members, and for supplying replenishment oil from the reservoir to the pressure chamber through a check valve. Thus the hydraulic lifter is adapted to allow prompt expansion of the pressure chamber volume and to permit a reduction in said volume, though offering resistance to the volume reduction in the latter case so that such reduction takes place at a slow rate. The hydraulic lifter utilizes these characteristics for quickly taking up play in the engine valve train, and thus functions as a lash adjuster for driving the valve train while resisting the force applied by a valve return spring.
The reservoir chamber in the plunger member typically is provided with a bag-shaped diaphragm the upper end of which is held in pressured contact with the inner wall of the reservoir chamber to effect a liquid-tight seal. The interior of the reservoir chamber is filled with a hydraulic fluid, namely oil, with the diaphragm serving as a flexible barrier between the oil in surface contact with one side thereof and the atmosphere on the other side. The diaphragm thus is free to expand and contract as the quantity of oil within the reservoir chamber changes with the telescoping movement of the plunger member.
Maximizing the lash adjustment capability of the hydraulic lifter is essential to obtain a lash adjuster having a high degree of reliability. Thus, the greater the change in the oil volume interiorly of the lifter reservoir, the better the lash adjusting performance, assuring excellent reliability even if some oil is lost through leakage. A requirement for obtaining such a large change in oil volume is to shape the inner wall surface of the reservoir chamber so as to allow a large change in the shape of the diaphragm as the diaphragm expands and collapses during operation of the hydraulic lifter.
The reservoir chamber in the conventional hydraulic lifter, however, has a cylindrically shaped inner wall over its entire length and is therefore readily contacted by the outer side of the diaphragm as the diaphragm expands in adapting itself to a change in chamber volume. Such contact between the diaphragm and the closely spaced reservoir chamber wall makes it impossible to achieve a large change in diaphragm shape, thereby placing a limitation upon the change in volume that can be brought to bear upon the oil within the reservoir. This prevents the attainment of a large lash adjustment capability and, hence, results in a lash adjuster having less than the desired reliability.